feat(protocols): add per-unit spec support to state protocol

[MacAttak]

Summary

• Adds unitId field to currentWork for tracking the active unit in multi-unit work
• Adds workDir and planned status to workUnits entries for per-unit artifact directories
• Establishes path resolution convention (unit-local vs design-level scopes) and gates reset rule

Acceptance Criteria

#	Criterion	Status
AC-1	unitId field in currentWork schema	PASS — state.md:17
AC-2	workDir field in workUnits entries	PASS — state.md:32
AC-3	planned status in workUnits enum	PASS — state.md:32,47
AC-4	shipped → building transition, no shipped → planning	PASS — state.md:62
AC-5	planning → building with sw-plan trigger	PASS — state.md:58
AC-6	Path Resolution Convention section	PASS — state.md:74-85
AC-7	Gates reset rule documented	PASS — state.md:72
AC-8	Single-unit backward compatibility	PASS — state.md:42-45
AC-9	workUnits null note preserved	PASS — state.md:42

Gate Results

Gate	Status	Findings (B/W/I)
gate-build	SKIP	0/0/0
gate-tests	SKIP	0/0/0
gate-security	SKIP	0/0/0
gate-wiring	SKIP	0/0/0
gate-spec	PASS	0/0/0

Context

This is Unit 1 of 4 in the per-unit-specs work. It establishes the state protocol foundation that Units 2-4 depend on.

Work units:

1. state-protocol ← this PR
2. sw-plan-rewrite
3. build-and-ship
4. downstream-and-docs